1. Field of the Invention
The present invention relates to a motor encoder for detecting the rotational position of a motor, and more particularly relates to a motor encoder capable of generating three-phase magnetic pole signals U, V, and W that differ in phase by 120° and indicate the pole positions of the motor rotor.
2. Description of the Related Art
FIG. 5 shows a commonly known three-phase AC servomotor. In an AC servomotor 100, a encoder 110 for detecting the position of the motor rotor, and a magnetic pole detection mechanism 120 for detecting the magnetic pole position of the motor are mounted on the rear end portion of a motor axle 102 on which a motor rotor 101 is integrally formed. A- and B-phase signals that differ in phase by 90° and are output from the encoder 110, and magnetic pole signals U, V, W that differ in phase by 120° and are output from the magnetic pole detection mechanism 120 are fed to a servo driver 130. The servo driver 130 is provided with a receiver circuit unit 131 for receiving these signals and a control circuit unit 132 for driving and controlling the AC servomotor 100 on the basis of commands from a host system 140 and the received signals.
The encoder 110 is provided with a rotational body 11 fixed coaxially with the motor axle 102, a pair of magnetic sensors 112A and 112B that face an external peripheral surface of the rotational body 11 via a fixed gap, and a signal processing circuit 113. The rotational body 11 is configured so that magnetic poles N and S are formed with a fixed pitch along a circular external peripheral surface, as shown in FIG. 6. The relative positions of the pair of magnetic sensors 112A and 112B are set so that A-phase and B-phase sinusoidal detection signals that differ in phase by 90° can be obtained in association with the rotation of the rotational body 111. The signal processing circuit 113 generates the A-phase and B-phase signals with the constant pulse cycles shown in FIG. 7 from the signals fed from the magnetic sensors 112A and 112B, and feeds these signals to the servo driver 130.
The magnetic pole detection mechanism 120 is provided with a magnetic pole rotational body 121 fixed coaxially with the motor axle 102, three magnetic sensors 122U, 122V, and 122W that face an external peripheral surface of the rotational body 121 via a fixed gap, and a signal processing circuit 123. The magnetic pole rotational body 121 is polarized in the same manner as the motor rotor 101, and is a quadrupolar magnet as shown in FIG. 8. The relative positions of three magnetic sensors 122U, 122V, and 122W are set so as to obtain a U-phase detection signal, a V-phase detection signal, and a W-phase detection signal in the form of sinusoidal waves that differ in phase by 120°, such as those shown in FIG. 9, in association with the rotation of the magnetic rotational body 121. The signal processing circuit 123 generates the U-, V-, and W-phase signals shown in FIG. 10 from the signals fed from the magnetic sensors 122U, 122V, and 122W, and feeds these signals to the servo driver 130.
Thus, an encoder and a magnetic pole detection mechanism are both required in a conventional three-phase AC servomotor, and these are disposed in the rear end portion of the motor axle. It would therefore be very advantageous for reducing the size and weight of the AC servomotor to provide these components in a small and compact configuration.